On Wednesday, the main challenger in the national presidential election, Edward Lowassa, said the problems in Zanzibar could be mirrored across the country.

The national electoral commission rejected his call for the entire election to be annulled.

The nationwide general elections have been the most tightly contested since independence in 1961.

Four opposition parties united to form the Ukawa coalition, hoping to end the governing CCM party's 54-year rule.

However, partial results show Mr Lowassa is trailing CCM presidential candidate John Magufuli.

Mr Magufuli has 59.2% of the vote compared with Mr Lowassa's 39.6% after results from 195 of Tanzania's 264 constituencies were announced by the national electoral commission, the AFP news agency reports.

Elections in Tanzania tend to be peaceful, except in Zanzibar where previous polls have been marred by violence following disputed results.

The BBC's Sammy Awami in Zanzibar while there is confusion about the announcement by Zanzibar Electoral Commission (Zec), it remains calm.

In a statement, the UK embassy in Tanzania also expressed concern about the scrapping of Zanzibar's elections.

"We note that all the international observers were impressed by the quality of the electoral process they witnessed. We call on Zec to resume the results tabulation process without delay," said the statement.